1. Field of Invention
This invention relates to high power efficiency audio amplifiers which have digital input and built in digital volume control.
2. Description of Prior Art
Prior art amplifier designs rely on Class-A, Class-AB (hybrid), or more recently, Class-D, Class-G, and Class-H, output stages to deliver high power to a speaker. In Class-A designs the output devices are biased into their most linear conductivity region. This allows for nearly perfect reproduction of the audio signal. However, a great price is to be paid in power efficiency. This is due to the fact that large DC bias currents flow constantly through the output devices resulting in continuous heat dissipation. Class-B amplifiers have higher efficiency than Class-A amplifiers at the expense of crossover distortion. This can be overcome by applying a small bias current to the output stage and thus create a hybrid output stage known as a Class-AB output stage. Class-AB output stages have nearly linear output and much improved power efficiency over Class-A designs.
Class-D amplifiers are rarely and only recently used for audio purposes. The most common use is for driving large power inductive loads such as synchronous motors and inverters. In Class-D amplifiers the output devices do not operate in their linear mode. Instead, these devices use pulse width modulation, where the duty cycle is modulated proportional to the amplitude of the input signal. Fourier analysis shows that the spectral output of this type of amplifier contains sine terms at the switching frequency and above and subharmonics representing the input signal. In order to use this type of amplifier the switching frequency terms are filtered away with a low pass (generally L-C type) filter. A Class-D amplifier can be implemented in a straight forward manner with analog circuits such as with a triangle wave generator, comparator, and high power output transistors as is shown in prior art. Class-G and Class-H amplifiers use variable tracking power supply rails in conjunction with Class-A, or Class-AB, output stages to achieve higher power efficiency then Class-A or Class-AB designs alone.
Disadvantages of the prior art include:
Pure Class-A designs are very power inefficient. To deliver high power to the load, the output stages may dissipate much greater than 50% of the total available power. PA1 Pure Class-B designs have crossover distortion and are rarely employed for audio. PA1 Class-AB designs improve on the power inefficiency of Class-A and the crossover distortion of Class-B, but are not as power efficient as the present invention. PA1 Pure Class-D designs have not been employed for audio because of their poor frequency response and difficulty with unwanted harmonic generation. PA1 Class BD, Class C, or class H designs may include variable rail voltages that track the desired output but do not offer the inherent volume control configuration of the present invention. The present invention's rail voltages are used for volume control not for tracking the desired audio output signal. PA1 Prior art cited does not make use of digital methods for generating PWM waveforms for use in audio amplifiers or in volume control. PA1 (a) very high power efficiency, PA1 (b) built-in volume control in the power electronics, PA1 (c) entire circuit can be viewed as digitally controlled analog multiplier circuit, and PA1 (d) digital inputs and control units which allow the use of digital domain control and filtering techniques.
The following U.S. Pat. Nos. 5,450,036, 5,424,684, 5,483,197, 5,508,658, 5,479,134, explicitly claim the use of an analog triangle wave, and comparator based techniques for PWM generation. U.S. Pat. Nos. 3,629,616 claims sawtooth wave for generation of PWM instead of triangle wave generation. U.S. Pat. No. 5,450,037 uses gradient detection to track the power supply rails to the output voltage. U.S. Pat. No. 5,442,317 discloses a switching power supply which tracks the audio output, but in this patent the supply voltage is independent of the volume level setting. U.S. Pat. Nos. 5,396,194 and 4,507,619 disclose exemplary example of a class BD audio amplifier with a fully tracking power supply system where the audio output is followed by a switching power supply and then reproduced by a linear output stage. U.S. Pat. No. 5,410,592 discloses a purely class D audio amplifier with a fixed design state variable feedback loop controller. U.S. Pat. No. 5,352,986 claims a 3rd or higher order control system for maintaining a desired voltage output at the load of a switching amplifier. U.S. Pat. No. 4,178,556 discloses an analog trap resonant filter on the output of a switching amplifier for removing the fundamental switching frequency from the output. U.S. Pat. No. 4,797,633 describes a staggered stage amplifier that uses peak detection to activate multiple stages, with each stage operating in linear mode.